Many small scale liquid dispensing devices, sometimes called micro-fluidic devices, are known. These devices include micro-electromechanical system (MEMS) devices, electrical semiconductor devices, and others. These devices are small, typically in the range of 500 microns down to as small as 1 micron or even smaller. These devices are important in a wide range of applications that include drug delivery, analytical chemistry, microchemical reactors and synthesis, genetic engineering, and marking technologies including a range of ink jet technologies, such as thermal ink jet and piezoelectric ink jet. Many of these devices have one or more layers that filter fluid flowing through the devices. These filters help keep nozzles and channels free of clogs caused by particle contaminants and air bubbles carried into the printhead from upstream liquid sources.
In some of these micro-fluidic devices, the filter layers are fabricated with polymer films and in others, the filter layer is made from a thin metal layer. Examples of polymer films useful for filter layers include polyimides, such as Kapton™ or Upilex™, polyester, polysulfone, polyetheretherketone, polyphenelyene sulfide, and polyethersulfone. Metal filters may be made from stainless steel, nickel electroformed screens, or woven mesh screens. The filter layer may be laser ablated or chemically etched to produce the filter pores. These pores are required to be smaller in diameter than the final aperture through which the fluid passes so they block the passage of contaminants that might block the final aperture. Ancillary structure may also be provided to redirect fluid flow to another portion of the filter in the event that a portion of the filter becomes clogged. In some micro-fluidic devices, the final aperture may be approximately 20-50 microns. Typically, the filter pores are 5-10 microns smaller than the final opening. Care must be taken in the pore production process to ensure the placement and sizing of the pores are within these relatively tight tolerance ranges.
After a filter layer is produced, it is mounted in a micro-fluidic device between two substrates, which are typically made of stainless steel or silicon. A number of methods are frequently used for the mounting of the filter. For example, a filter may be brazed, ultrasonically bonded, or anodically bonded with the lack of adhesive between the substrates. Alignment of the filter with an inlet in a substrate on one side of the filter and with an outlet in a substrate on the other side of the filter must be accomplished with some precision. Otherwise, fluid flow through the filter may be impeded.
A filter layer may alternatively be mounted between substrates by applying adhesive to both surfaces of the filter layer before aligning the filter layer between two substrates. Application of the adhesive requires attention as the adhesive may clog pores in the filter if the adhesive directly contacts the filter pores. Additionally, the adhesive is typically applied to one surface of the filter layer or the mating substrate, and then the filter layer is pressed against a substrate. After the adhesive is cured, adhesive is then applied to the other filter surface or other substrate, the other substrate is pressed against the filter layer surface, and the adhesive cured. Thus, assembling a micro-fluidic device with a filter layer requires separate adhesives, assembly steps, and curing steps for each interface.
While the above-described processes are effective for producing and mounting filter layers in micro-fluidic devices, they do require a number of distinct steps and careful control. Accordingly, development of more robust processes for making and mounting filters in micro-fluidic devices is desirable.